Fuel inside a vehicle fuel tank regularly produces a vapor which collects above the fuel and, if not released, causes an increase in tank pressure. Control of this fuel vapor is necessary to prevent overfilling and overpressurization of the tank. In addition, environmental concerns and regulations dictate that the fuel vapor be controlled so that it does not escape to the atmosphere.
Several methods and devices have been used to control and recapture fuel vapor, most employing a carbon canister to trap the vapor. These devices differ mainly in the type of valve used to route the vapor to the carbon canister and the mechanism for triggering the valve. Many such valves are triggered by insertion of a fuel pump nozzle or by removal of the gas cap during refueling. These valves include, for example, a vacuum actuated piston vent valve (U.S. Pat. No. 5,014,742, Covert, et.al.), a solenoid assisted float valve (U.S. Pat. No. 5,054,528, Saitoh), a two-way valve (U.S. Pat. No. 4,343,281, Uozumi et.al.), and a check valve (U.S. Pat. No. 4,040,404, Tagawa).
The valve type disclosed in U.S. Pat. No. 5,099,880 (Szlaga et.al.) uses spring-biased diaphragm assemblies to maintain sufficient tank pressure during refueling to prevent overfill. The diaphragm assemblies are arranged such that fuel vapor remains in the tank while refueling, but is routed to a canister when the filler cap is replaced and tank pressure exceeds a preset value. Several chambers in the valve allow the diaphragm assemblies to route the fuel vapor to the canister. However, operation of such a valve will fail if an equalization orifice (for example, bleed passageway 56 in FIG. 1 of the '880 patent) between chambers on either side of a diaphragm becomes blocked. When equalization of pressure between chambers does not occur, opening of the diaphragm may be impeded, thus hindering the vapor flow from the tank to the canister. Tank pressure could then exceed design limits. Such a valve design does not allow for the possibility of contamination and the consequent tank overpressurization.
Another valve utilizing a diaphragm arrangement for venting a fuel tank was disclosed in U.S. Pat. No. 5,054,508 (Benjey). This valve, however, has no mechanism to relieve tank pressure should contamination or malfunction prevent normal operation.